A-FRAME DAMS. 



25? 



1 



has but commenced. It is therefore more rapidly raised ; the lowering is also more 

 rapid. 



There is nothing left standing to catch drift after the lowering begins. 



There are no extra parts to care for when lowering or when not in use. 



There is no danger to operatives in the maneuvers. 



As there is no double construction, the foundation is narrow; hence the cost is 

 reduced. 



The leakage will be very little, as there are few joints. 



If submerged, it will act as a fixed dam without injury to itself. 



A weir of this character, 120 feet 

 long and 13 feet 2 inches high, is in 

 use at Dam No. 6 on the Ohio River. 



Calculations. -- The strains in a 

 trestle of the A-frame style are com- 

 posed of compression in the down- 

 stream leg and tension and bending 

 combined in the up-stream leg. It 

 may be mentioned that as the height 

 of these trestles is increased, neces- 

 sitating an increased proportion in the 

 width of the base, the resultant of the 

 water pressure will fall within the lat- 

 ter, thus removing any upward pull on 

 the anchorages. 



Let ABC (Fig. 22) represent an A-frame of width of face w, supporting a head 

 of water H, and with legs inclined at angles a and /? as shown. 



IT 



The pressure P on the up-stream leg is wH sec a . X 62 Ibs. 



2 



Taking moments about B, we have 



PX 



H sec a 



AC X BD, or AC = 



PH sec a 



Similarly, taking moments about C, if CE is the perpendicular from C to the 

 direction of P, 



PXCE 



P X CE = AB X CF, or AB = 



CF 



These equations will give the direct strains in AC and AB, but AB has to support 

 in addition the bending from the water. The point of maximum bending for a beam 



TT 



supporting water level with its top and on one side only occurs at ?=, H being as given 

 above. The moment thus is 



